Landing Gear Mechanism for Model Airplane

ABSTRACT

A radio controlled model airplane has landing gear including a wheel rotatably mounted on a strut. The wheel and strut are retractable into the airplane wing. The strut is angled forwardly relative to the wing surface where the strut is attached such that the wheel is forward of the airplane center of gravity just prior to takeoff and touchdown. The landing gear includes a control assembly mounted on the wing. The control assembly includes a housing and a pivot pin on which the strut is mounted. The pivot pin is rotatable and pivotable around one of its ends. The control assembly also includes a cam which causes the pivot pin to pivot upon rotation, thereby causing the strut to move through a plane oblique to the wing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/165,208 filed Mar. 31, 2009, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to landing gear for radio controlled model airplanes, and more particularly to an improved structure for retractable landing gear.

Most radio controlled model airplanes are equipped with landing gear to facilitate take-offs and landings. Typically, the landing gear includes a wheel supported on the end of a strut. Generally, the landing gear struts are perpendicular to the airplane wings to which they are attached. Some landing gear is permanently fixed in position. Other landing gear is retractable and folds up into the wing after take off. Retractable landing gear is desirable in many types of radio controlled airplanes for reasons such as authenticity, maneuverability and handling.

The center of gravity or balance point of many radio controlled model airplanes is substantially directly over the wheels just before takeoff and just prior to touchdown. This can cause the plane to teeter on the wheels and often creates “nose over” conditions during both take-offs and landings. In a “nose over” condition, the tail of a radio controlled model airplane will rise, and the nose of the airplane will hit the ground and stall the engine. Most runways are grass, causing increased drag and exacerbating the nose over condition because of the higher rolling resistance.

To avoid a nose over condition during take-off, a radio controlled model airplane can be positioned on a runway, and then be caused to “blast off,” which is accomplished by immediately applying both full throttle and up elevator (the control surface that makes the plane go up or down). The plane blasts down the runway a few feet and jumps skyward like a rocket. This can be dangerous, as the model airplane may be somewhat uncontrollable for the first few seconds of flight. This type of take-off also looks unnatural. Landings are easier than take-offs, but as a radio controlled model airplane touches down and its speed decreases, the airplane can stop in a “nose over” condition.

One way to alleviate the problems caused by “nose over” conditions is to used fixed landing gear struts angled forwardly, as opposed to generally vertically oriented, therefore positioning the wheels forward of the center of gravity. In these fixed landing gear type of radio controlled model airplanes, the “nose over” condition does not tend to occur as frequently.

Currently available mechanisms for retractable landing gears are simple. They generally consist of an actuating arm and a pivot on top of the strut. The pivot has a pin about which the strut rotates. The pivot is housed in a small box or case. The mounting bores for the pin, and therefore the pin axis, are aligned (i.e. parallel) to the fuselage of the plane, restricting the strut and wheels to swing or parallel to the wing. No commercially satisfactory design has been made to adequately retract landing gear having forwardly angled struts.

SUMMARY OF THE INVENTION

My invention is a retractable landing gear assembly for model airplanes having a strut angled forwardly relative to the wing surface where the strut is attached. The landing gear includes a control assembly mounted on the wing. The control assembly causes the strut to move through a plane oblique to the wing. Preferably, the control assembly includes housing and a swivel to which the strut is attached. The swivel is mounted for rotation in the housing. The swivel has a fixed end and a free end which allows pivoting relative to the housing. The swivel includes a cam which pivots the free end of the swivel from a first position to a second position upon rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art radio controlled model airplane.

FIG. 2 is a perspective view of the underside of the wings of the model airplane of FIG. 1.

FIG. 3 is a perspective view of a model airplane equipped with the landing gear of the present invention.

FIG. 4 is an isometric view of the landing gear control assembly of the present invention.

FIG. 5 is a cross-sectional view of the landing gear control assembly of FIG. 4.

FIGS. 6, 7, 7A, and 7B are cross-sectional views of components of the assembly of FIG. 4.

FIG. 8 is schematic view of the landing gear mechanism of FIG. 4.

FIG. 8 a is a schematic view of an alternative construction of the swivel of FIG. 8.

FIG. 9 is a schematic view of, an airplane landing gear arrangement having two landing gear assemblies.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a radio controlled model airplane 10 has a fuselage 11 with a longitudinal central axis 12. Wings 14 extend transversely to the fuselage, generally along an axis 15. Airplane 10 is a “tail-dragger”, having a small fixed rear landing wheel 16 is attached to the rear portion of the fuselage. However, this invention may also be suitable for other airplane types, such as a “tri-gear” type having two wheels generally under the wings and third wheel under the nose of the aircraft. Landing gear 20 is attached to the underside of each wing. Each landing gear includes a strut 22 and a wheel 24 rotatably attached to each strut. Strut 22 lies on a vertical axis 26 witch in most prior art designs is substantially perpendicular to fuselage axis 12. The embodiment shown in FIG. 1 has struts which are tilted slightly forward relative to an axis 28 perpendicular to the fuselage axis 12. Generally, the forward tilt angle 29 of a retractable landing gear is in the range of two to three degrees or less because of the difficulties in retracting landing gear having a larger forward tilt. Even with this slight forward tilt, the wheels are generally directly over the center of gravity of the airplane just prior to takeoff and touchdown.

Referring to FIG. 2, the underside of wings 11 have wheel and strut wells 30 for receiving landing gear in the retracted position. FIG. 2 shows one landing gear in the retracted position and the other in the extended position. The underside surfaces of wings 11 are curved from front to back as viewed along the longitudinal fuselage axis 12. The wing surfaces 32 adjacent the wells 30 may be tilted slightly upward (when viewed in FIG. 1). The location of the wheel wells on the forward portion of the wing provides for the slight forward tilt angle 29 of the struts. The pins about which the struts rotate are generally in the same plane as the wing surface adjacent the wells so that the struts fit flush into the wells. But for the slight tilt of the wing surface, the struts retract in a plane essentially parallel to the wing axis 15,

FIG. 3 shows airplane 10 with the landing gear 40 of the present invention. Landing gear 40 includes a strut 42 and wheel 44. The strut 42 lies on a axis 46 which is tilted forward an angle 49 of about 9.5 degrees relative to the prior art strut axis 26. The amount of forward angle may vary from about five to about twenty degrees, but 9.5 degrees has been found to be optimum for the tail dragger type of the illustrated embodiment. Of course, the optimum angle may change with other variables such as the length of the strut.

FIG. 4 shows the landing gear control assembly 50 having a housing preferably made of a rigid molded plastic in two pieces 52, 54. A swivel 80 inside of the housing is rigidly connected to strut 42 which is shown in the extended (wheel down) position. The landing gear control assembly 50 is mounted in the wing 14, preferably in the outboard end of the well 30.

Referring also to FIG. 5, control assembly 50 shown in the retracted position. Control assembly 50 includes a swivel 80 into which is press fit a pivot pin 70 so that the swivel and pivot pin are rigidly connected. Alternatively, the pivot pin can be formed as an integral part of the swivel. Actuator shaft 60 is also mounted in the housing in a bushing 62 having a through bore. The actuator shaft 60 is movable in either direction along its longitudinal axis by an electric or pneumatic servo motor as is well know in the art. The control assembly also contains a wiper member 66 which rides in a track 68 in the housing. Wiper member 66 is fixed to the end of actuator shaft 60, such as with a cap screw 64, or in any other well known manner.

A bushing 63 is positioned in a housing bore opposite bushing 62. Bushing 63 is solid except for a conical drill indentation 65 which provides clearance for the cap screw 64, even though such clearance is generally not needed to allow a full extension of the actuator shaft 60. By reversing the solid bushing 63 and the bored busing 62, the landing gear may operate with an outboard servo instead of an inboard servo as will be described later.

Swivel 80 includes an integral cam 90 having upper and lower cam surfaces 92, 94. Swivel includes an integral fork 96 engageable with wiper 66 upon axial movement of the actuator shaft 60 in either direction. Upon movement of wiper 66 from one end of the housing as seen in FIG. 5 to the other end, the swivel will rotate 85-90 degrees, depending on the particular application, with the shift fork walls being engaged by the wiper in either direction. Alternatively, the fork 96 could be on the wiper, with a cooperating finger on the swivel to accomplish the same result. Swivel 80 includes surfaces 93 and 97 which engage the housing as will be described later.

FIG. 6 illustrates the rear housing portion 52 having an inner rear surface 57. Bore 72 accepts and retains the fixed end of pivot shaft 70. Because the opposite end of shaft 70 pivots through an arcuate path, bore 72 has a slightly, oblong shape to accommodate very limited pivoting of the shaft within the bore 72. Bore 72 is formed by drilling a first hole and then making a second drill pass at an angle of about 80.5 degrees relative to the initial drill hole. This provides a tight fit relative to the horizontal direction as viewed in FIG. 6 and allows a limited pivoting in the vertical direction as viewed in FIG. 6.

Front housing portion 54 having an inner front surface 53 is shown in FIG. 7. Elongated bore 74 accepts and retains the free end of the pivot pin 70 and allows only vertical movement of the free end as viewed in FIG. 7. FIG. 7A it can be seen that the bottom wall of the bore 74 is drilled generally parallel to the bottom wall of the housing, while the upper wall of the bore is drilled at an angle 75 of about 9.5 degrees relative to the bottom wall of the bore to allow for pivoting of the pivot pin 70 within the bore 74. In a preferred embodiment, the horizontal dimension of both bores 72, 74 is about ⅜ inches, the same as the diameter of the pivot pin 70. This prevents substantially all lateral movement or “play” of the pin within the bores. The free end of pivot pin 70 travels only about 0.130 inches as the landing gear moves from a fully extended to a fully retracted position.

FIG. 7B shows a cut away side view of housing portion 54 along line B-B of FIG. 7. Also referring to FIG. 8, upper and lower cam rails 82, 84 are integrally formed in housing portion 54. Cam surfaces 92, 94 engage rails 82, 84, respectfully. Cam surfaces 92, 94 are generally in constant contact with the respective housing rails 82, 84 due to very small clearances. However, only one cam surface exerts any significant force on a rail at any given time, depending on the direction of rotation of the swivel, i.e. whether the landing gear is being moved from a retracted or and extended position.

Referring to FIG. 8, swivel 80 is shown in the extended position for takeoff and landing, the forward direction of airplane travel indicated by arrow 99. It can be seen that swivel surface 97 is tapered to allow free movement of the swivel as it moves to and from the extended and retraced positions. Swivel 80 generally rides against the housing walls 53 and 57 throughout a full range of swivel pivoting. As such, the housing may absorb any shock forces, such as may be caused by a hard landing. Swivel 80 includes a bore 81 into which is press fit a strut 46. Pivot pin 70 has a tapped axial bore 71 to accept a set screw 95 which holds the strut in the swivel.

FIG. 8 a shows an alternatively constructed swivel 80′. Swivel 80′ has an integral pin 70′. Swivel 80′ is made of one piece of aluminum machined with CNC equipment.

FIG. 9 shows an airplane landing gear arrangement with two landing gear assemblies 50, 50′ operated through two actuator shafts 46, 46′, respectively. An inboard servo motor 100 is located between the assemblies 50, 50′ on or in the fuselage. The servo is connected to a′ rotatable cross arm 101 which in turn is connected on opposite ends to the actuator shafts 46, 46′ through a clevis (not shown) whereby rotation of the cross member creates reciprocating movement of the actuator shafts as will be appreciated buy those skilled in the art. The clevis facilitates angular movement between the cross arm and the actuator shaft as is well known in the art.

Alternatively, two servo motors can be used, one for the landing gear of each wheel. Generally two servos would be used only if the airplane is not suited to accommodate a single inboard servo for any reason, for example limited inboard space. Two inboard servos could be used, but more typically two outboard servos would be used. The control assembly 50 described herein can easily be adapted for an outboard servo motor by reversing the bushings 62, 63 and flipping the actuator shaft 60.

In summary, when the landing gear is in the retracted, wheel up position, the pivot pin 70 is in essentially the same position as a pivot pin used in prior art devices, allowing the strut and wheel to align with the wing surface and landing gear well for a flat fit. However, in the extended, wheel down position, the cam 90 causes one end of the pin 70 to raise and lower in the front bore 74 while the other end remains stationary. The forward tilting of the pivot pin 70 causes the strut and wheel to tilt forward. The cam has a flowing angle to accommodate the gradual tilt as the landing gear moves to an from extended and retracted positions. The relief angles on the swivel surface 97 also accommodate this movement.

In a preferred embodiment, the control assembly 50 components are made of lightweight metal such as aluminum. However, the housing 52, 54 and the swivel 80 may be made from plastic, with the cam surfaces, rails, etc. being precisely machined with CNC equipment.

The landing gear of the present invention may be used as either original equipment, such as in a model airplane kit, or as a retrofit assembly. For a typical model airplane, an optimum forward tilt for a strut is about nine and one-half degrees from its original position. For a typical strut length of about 6.5 inches and wheel diameter of about 3 inches, this amount of tilt will move the wheel forward slightly over one inch (1.0672 inches). This forward movement is sufficient to keep the airplane center of gravity behind the wheel just priot to takeoff and touchdown of the airplane.

The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. 

1. A model airplane comprising an elongated fuselage having a longitudinal axis, a wing oriented transversely to the fuselage along an axis generally transverse to the fuselage axis, and a retractable landing gear assembly attached to the airplane, the landing gear assembly comprising an elongated strut having a longitudinal axis with one end of the strut pivotally mounted on the airplane the other end of the strut rotatably attached to a wheel, wherein the strut is movable from a retracted position in which the strut axis is substantially parallel to the wing axis to an extended position in which the strut axis is substantially oblique to the wing axis.
 2. A model airplane as defined in claim 1 wherein the strut is pivotally mounted on the wing, wherein the wing surface defines a landing gear well, and wherein the strut axis is substantially oblique to the plane of the wing surface adjacent the well when the strut is in the extended position.
 3. A model airplane as defined in claim 1 wherein the strut axis when, the strut is in the extended position is substantially perpendicular to the wing axis when viewed along an axis parallel to the fuselage axis, and skewed with respect to the fuselage axis and wing axis when viewed along an axis parallel to the wing axis.
 4. A model airplane as defined in claim 3 wherein the strut axis is skewed about 9.5 degrees with respect to the fuselage axis and wing axis.
 5. A model airplane as defined in claim 1 comprising two struts, wherein each of the two struts is operated by a servo positioned inboard of each strut.
 6. A model airplane as defined in claim 1 having radio controls and a servo to move the strut to and from the extended and retracted positions.
 7. A model airplane as defined in claim 1 wherein the airplane is a tail dragger type airplane.
 8. A retractable landing gear control assembly for a model airplane, the assembly comprising: a housing having a cam rail, a swivel having a substantially fixed end mounted in the housing and a free end mounted in the housing for pivotal movement about the fixed end, the swivel including cam engageable with the cam rail, an actuator engageable with the swivel to rotate the swivel, wherein upon rotation of the swivel, the cam causes the free end of the pivot pin to move from a first position to a second position.
 9. A retractable landing gear control assembly as defined in claim 8 wherein the housing further comprises a second cam rail, the cam engageable with the second cam rail.
 10. A retractable landing gear control assembly as defined in claim 8 wherein a landing gear strut is attached to the swivel.
 11. A retractable landing gear control assembly as defined in claim 8 wherein the actuator comprises a shaft mounted in the housing transverse to the pivot pin.
 12. A retractable landing gear control assembly as defined in claim 9 further comprising a wiper affixed to the actuator shaft, the wiper engageable with the swivel.
 13. A retractable landing gear control assembly as defined in claim 10 wherein one of the wiper and swivel includes a fork engageable with the other one of the wiper and swivel.
 14. A retractable landing gear control assembly as defined in claim 11 wherein the wiper rides in a housing track.
 15. A retractable landing gear control assembly as defined in claim 8 wherein the actuator comprises a shaft having a longitudinal axis, and further comprising a servo motor for moving the actuator shaft along the longitudinal axis.
 16. A retractable landing gear assembly as defined in claim 13 wherein the housing further comprises bushings for facilitating axial movement of the actuator shaft relative to the housing.
 17. A retractable landing gear assembly as defined in claim 8 wherein the swivel comprises a pivot pin.
 18. A retractable landing gear assembly as defined in claim 17 wherein the pivot pin is integrally formed with the swivel.
 19. A model airplane comprising a longitudinally oriented fuselage, a wing oriented transversely to the fuselage, and a retractable landing gear assembly attached to the wing, the landing gear assembly comprising a strut mounted on the wing, a wheel rotatably attached to the strut, and a pivot pin having a first end and a second end, wherein the strut is rotatable around the pivot pin, and wherein the first end of the pivot pin is substantially fixed relative to the wing and the second end of the pivot pin is movable with respect to the wing.
 20. A model airplane as defined in claim 19 wherein the airplane is a tail dragger type. 